Grauer’s gorillas have been declared critically endangered after a population decline from 16,900 to 3,800 over a 20-year period.

The International Union for Conservation of Nature (IUCN) released Sunday an updated Red List of Threatened Species, which makes designations from "least concern" to "extinct" for more than 80,000 plant and animal species.

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“We live in a time of tremendous change and each IUCN Red List update makes us realize just how quickly the global extinction crisis is escalating,” IUCN Director General Inger Andersen said. “It is our responsibility to enhance our efforts to turn the tide and protect the future of our planet.”

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The Grauer’s gorilla along with the mountain gorilla are the two subspecies of eastern gorillas, which are considered the world's largest apes and live in the forests of Uganda, Rwanda, and the Democratic Republic of Congo.

The mountain gorillas population only consists of about 800 individuals, but the subspecies has show signs of rebounding, the IUCN said.

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Four of the six great apes – eastern gorilla, western gorilla, Bornean orangutan, and Sumatran orangutan – are now listed as critically endangered, just one step away from extinction.

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The chimpanzee and bonobo, the other two great apes, are currently listed as endangered.

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Chimpanzee

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Bonobo

The main threat to the eastern gorillas is hunting and habitat disturbance from conflicts in the area, while orangutans in Malaysia and Indonesia are threatened by the rapid loss of habit for the cultivation of palm oil.